Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. Conventional pacemakers are implanted subcutaneously or submuscularly in a patient's chest and have leads threaded intravenously into the heart to connect the device to electrodes used for sensing and pacing. A programmable electronic controller causes the pacing pulses to be output in response to lapsed time intervals and sensed intrinsic electrical activity (i.e., heart beats not as a result of a pacing pulse). The most common condition for which pacemakers are used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sinus node dysfunction represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate.
Pacing therapy can also be used in the treatment of heart failure, which refers to a condition where an abnormality of cardiac function causes cardiac output to fall below a level adequate to meet metabolic demand. Cardiac failure can be due to a variety of etiologies with that due to ischemic heart disease being the most common where a diminished coronary blood flow decreases the contractility of the myocardium. Some heart failure patients suffer from some degree of AV block or are chronotropically deficient such that their cardiac function can be improved with conventional bradycardia pacing. It has also been shown, however, that some heart failure patients suffer from intraventricular and/or interventricular conduction defects (e.g., bundle branch blocks) such that their cardiac outputs can be increased by improving the synchronization of right and left ventricular contractions with electrical stimulation. Other conduction defects can occur in the atria. Cardiac rhythm management devices have therefore been developed which provide pacing stimulation to both atria or to both ventricles in an attempt to improve the coordination of cardiac contractions, termed cardiac resynchronization therapy.